How to get access to WriteableBitmap.PixelBuffer pixels with C++?

Question

There are a lot of samples for C#, but only some code snippets for C++ on MSDN. I have put it together and I think it will work, but I am not sure if I am releasing all the COM references I have to.

Answer 1

Your code is correct--the reference count on the IBufferByteAccess interface of *buffer is incremented by the call to QueryInterface, and you must call Release once to release that reference.

However, if you use ComPtr<T>, this becomes much simpler--with ComPtr<T>, you cannot call any of the three members of IUnknown (AddRef, Release, and QueryInterface); it prevents you from calling them. Instead, it encapsulates calls to these member functions in a way that makes it difficult to screw things up. Here's an example of how this would look:

// Get the buffer from the WriteableBitmap:
IBuffer^ buffer = bitmap->PixelBuffer;

// Convert from C++/CX to the ABI IInspectable*:
ComPtr<IInspectable> bufferInspectable(AsInspectable(buffer));

// Get the IBufferByteAccess interface:
ComPtr<IBufferByteAccess> bufferBytes;
ThrowIfFailed(bufferInspectable.As(&bufferBytes));

// Use it:
byte* pixels(nullptr);
ThrowIfFailed(bufferBytes->Buffer(&pixels));

The call to bufferInspectable.As(&bufferBytes) performs a safe QueryInterface: it computes the IID from the type of bufferBytes, performs the QueryInterface, and attaches the resulting pointer to bufferBytes. When bufferBytes goes out of scope, it will automatically call Release. The code has the same effect as yours, but without the error-prone explicit resource management.

The example uses the following two utilities, which help to keep the code clean:

auto AsInspectable(Object^ const object) -> Microsoft::WRL::ComPtr<IInspectable>
{
    return reinterpret_cast<IInspectable*>(object);
}

auto ThrowIfFailed(HRESULT const hr) -> void
{
    if (FAILED(hr))
        throw Platform::Exception::CreateException(hr);
}

---

Observant readers will notice that because this code uses a ComPtr for the IInspectable* we get from buffer, this code actually performs an additional AddRef/Release compared to the original code. I would argue that the chance of this impacting performance is minimal, and it's best to start from code that is easy to verify as correct, then optimize for performance once the hot spots are understood.

Answer 2

This is what I tried so far:

// Get the buffer from the WriteableBitmap
IBuffer^ buffer = bitmap->PixelBuffer;

// Get access to the base COM interface of the buffer (IUnknown)
IUnknown* pUnk = reinterpret_cast<IUnknown*>(buffer);

// Use IUnknown to get the IBufferByteAccess interface of the buffer to get access to the bytes
// This requires #include <Robuffer.h>
IBufferByteAccess* pBufferByteAccess = nullptr;
HRESULT hr = pUnk->QueryInterface(IID_PPV_ARGS(&pBufferByteAccess));

if (FAILED(hr))
{
    throw Platform::Exception::CreateException(hr);
}

// Get the pointer to the bytes of the buffer
byte *pixels = nullptr;
pBufferByteAccess->Buffer(&pixels);

// *** Do the work on the bytes here ***

// Release reference to IBufferByteAccess created by QueryInterface.
// Perhaps this might be done before doing more work with the pixels buffer,
// but it's possible that without it - the buffer might get released or moved
// by the time you are done using it.
pBufferByteAccess->Release();